I am sure that this question has no "right" answer, at least there will be no grades given. Last night I submitted a slide of Iguazu Falls to a local camera club competition. It was probably taken at 1/250th second giving a somewhat rigid appearance, yet I believe that it shows the dynamics of these powerful cataracts. One judge felt that it should have been given a much longer exposure to soften the effect to the viewer. I like the long exposures for small deep-woods waterfalls (they often are in shade giving little choice). The diaphanous image enhances the impact and tranquility that I wish to remember. I expect to do some more falling water pics in the very near future, and would welcome any thoughts.